Philly,
Have you and David ever discussed therapy? I am an advocate of therapy, it helped me more than I can say. It sounds like he may not be open to it though.
As a patient I can say, when people kept asking me about it or how I was I would always say "I am fine" because I HATED people asking me. Then I would come home and be mad and sad. I couldn't stand people asking me because they made me think about it when I didn't want to. Then, they would look at me like I just ordered my coffin which made me feel like crap. They would look at me with this look of pity that I couldn't stand. It made me very angry. There are only a handful of people I can actually talk to. My husband is one of them and I'm sure you are one of those people for David. I always think it's easier to be the patient than the caregiver so I feel for you. If he is himself with you, regardless of how he is, it's because he feels comfortable with you.
I wish you luck
xoxo